Technical Field of the Disclosure
The present embodiment is related in general to propellants, and in particular to a variety of improvements to previously disclosed electrically controlled solid propellants, wherein said propellants are in a liquid state.
Description of the Related Art
Gas generating compositions are herein defined as any material, which stores chemical energy in a fixed volume. Explosives, propellants, pyrotechnics and other gas generating compositions are examples of materials, which may vary significantly in their performance. Reaction in these compositions generally results from either shock or heat. Explosives and propellants may also be thought of simply as a means of storing gas as a solid. Pyrotechnics typically release much of their energy as heat. Energetic gas generating materials often consist of fuels and oxidizers, which are intimately mixed. Incorporating fuels and oxidizers within one molecule or through chemical and physical mixtures of separate fuel and oxidizer ingredients is generally sufficient to mix the composition. The material may also contain other constituents such as binders, plasticizers, stabilizers, pigments, etc.
Gas-generating propellant compositions have numerous applications such as rocket propulsion systems, fire suppression systems, oil field services, gas field services, mining, torpedoes, safety airbag systems, and other uses where quickly expanding gas is employed for its work output. Often in these applications, it is desirable to control the ignition, burn rate, and extinguishment of a propellant by the application of an electrical current.
One of the major technical drawbacks to solid propellants has always been the lack of throttle control and the ability to restart motors once ignited. Conventional solid propellants also continue to be dangerous to manufacture, transport, and use since they are subject to accidental ignition from flames or sparks. Once ignited, conventional solid propellants lend themselves to be only minimally controlled, are not easily extinguished or restarted. These characteristics limit the function and increase the cost of propellant systems. Typically, such conventional propellants have Department of Transportation (DOT) shipping hazard classifications of Class 1.1 to 1.3 Explosives. In many of these instances, an electrically controlled propellant may allow the duration and burn rate of the propellant to be precisely controlled, while additionally allow cost reductions, mission flexibility, all with reduced hazard classifications simplifying supply or transport.
In some military, space and commercial applications, a smokeless or otherwise low signature propellant is desired. Such formulations typically do not contain metal fuels or chlorine based oxidizers such as ammonium perchlorate. Conventional formulations utilize oxidizers referred to as nitramines in the place of ammonium perchlorate. In other applications, high burn rate composites are required, in which case nitramines (RDX, HMX) in combination with nitroglycerin or nitrocellulose are used. These types of propellants are generally considered class 1.1 Explosives, which require added safety precautions in production, shipping and storage. In addition, high specific impulse (Isp) propellants are usually formed with ammonium perchlorate composites containing aluminum. These types of composites generate smoke from both the aluminum combustion and the hydrochloric acid generated when the composition interacts with moisture. Finally, all of the current propellants are spark-sensitive, meaning accidents occurring from stray static charges may at any time cause ignition of the propellants during manufacturing.
In the past, polytetrafluoroethylene (PTFE) and other substances have been used as electrically controlled propellants, but these prior art propellants suffer from two significant drawbacks. First, they often do not extinguish as quickly as desired after the electrical current has stopped. Second, these propellants provide none of their own energy, since all the energy for propellant gas generation comes from the electrical energy source. Further, compositions made from fluorocarbons and active metal fuels generally require the use of a flammable solvent in manufacturing, which can result in spontaneous ignition and disastrous results. Once blending has been achieved, the flammable solvent must be removed and recovered, adding to the cost of the manufacturing process.
In contrast to conventional liquid propellants, conventional solid propellants combusted with electric power traditionally require high voltage (in the range of kilovolts) pulse discharges, resulting in ablation of the propellant surface to produce ionizing gas species that are then accelerated by an electromagnetic field. Propellants such as these suffer from two serious drawbacks. First, conventional solid propellants will not extinguish immediately after the cessation of electrical current, thereby reducing the precision of control. Second, non-energetic solid propellants provide none of their own thrust, since the major portion of the thrust is generated by acceleration of the gas generation ions formed from the electrical energy source. In certain instances, it would be beneficial to directly generate thrust from the gas generated by the chemical combustion of the propellant. To date, neither a liquid, solid or gas phase propellant exists that can provide a dual purpose propulsion system, providing chemical thrust for more rapid movement and hazard avoidance combined with the potential for low thrust, high specific impulse applications.
One of the existing electrically controlled propellants comprises a binder, an oxidizer, and a cross-linking agent. The boric acid (the cross-linking agent as physical properties improvement additive) has been found to physically and chemically interact with the high molecular binder used to make the propellant, thereby improving the ability of the composition to withstand combustion without melting. The propellant also may include 5-aminotetrazole (5-ATZ) as a stability-enhancing additive. The binder of the propellant may include polyvinyl alcohol (PVA) and/or the co-polymer of polyvinyl alcohol/polyvinyl amine nitrate (PVA/PVAN). However, sustained combustion at pressures less than 200 psi without the application of continuous electrical power input is not generally achievable using the propellant. Further, burn rates at pressures above 200 psi (at which the propellants would sustain combustion) are lower than conventional composite solid propellants.
Another existing electrically controlled propellant comprises an ionomer oxidizer polymer binder, an oxidizer mix including at least one oxidizer salt and at least one eutectic material, and a mobile phase comprising at least one ionic liquid. The PVAN polymer in the propellant may be of medium (>100,000) to high molecular weight (<1,000,000). The propellant also may include the controlled cross-linking of the polymer through the use of epoxy resins, the use of a moisture barrier coating, and the addition of combustion additives such as Chromium III and polyethylene glycol polymer. However, under certain circumstances the propellant can melt or soften during combustion, thereby decreasing its effectiveness. More particularly, melting can undermine the ability of the propellant to be used in situations where the propellant must be ignited and extinguished multiple times. In addition, the fluid phase of the propellants in this application has sufficient volatility to slowly evaporate from the surface of the propellant, making its application unsuitable for use in the vacuum of space.
Another existing composition is capable of producing either solid propellant grains, liquid or gel monopropellants, all of which are electrically ignitable and capable of sustained controllable combustion at ambient pressure. Applications for the compositions include among other applications use in small micro-thrusters, large core-burning solid propellant grains, shaped explosives charges for military application, and pumpable liquid and gel monopropellants or explosives for military, commercial mining, or gas and oil recovery. In alternative embodiments, the above compositions may also incorporate an nitrate polymer, burn rate modifiers, and/or metal fuel(s). The High Power Electric Propulsion (HiPEP) formulation makes it possible to ignite and sustain combustion at ambient and vacuum conditions without continuous electrical power while providing faster burn rates.
Various other pyrotechnic compositions exist that include metastable intermolecular composites (MICs), providing liquid oxidizers in place of traditional solvents, thus eliminating the need for solvent extraction. The liquid oxidizer serves as a medium in which to suspend and grow the 3D nanostructure formed by the cross linked polymer (PVA). As a consequence, the 3D nanostructure entraps the liquid oxidizer, preventing it from evaporating and thereby eliminating the need for solvent extraction; and preserves the 3D nanostructure shape. Further, the liquid oxidizer matrix produced provides a mechanism through which ignition and combustion may be controlled. The material combustion rate may be adjusted/throttled through adjustments in the amount of the electrical power supply and may even be extinguished by complete removal of the electrical power supply. Repeated on/off ignition/extinguishment is possible through repeated application and removal of electrical current.
While the propellants disclosed above provide many advantages such as the ability to electrically control both ignition and extinguishing of the propellant, as well as multiple controlled initiation and extinguishing cycles, these electrically controlled propellants (ECPs) may still be improved upon. Specifically, the ECPs previously disclosed can be improved through the selective formulation modifications resulting in the propellants taking on a liquid form.
Based on the foregoing there is a demonstrable need for a liquid composition, which may be electrically initiated and controlled. Such a needed composition would have the ability to electrically control both ignition and extinguishing of the propellant, as well as provide multiple controlled initiation and extinguishing cycles. The liquid composition would comprise additives that act as viscosity modifiers for selective adjustment of the viscosity and flow characteristics (rheology). The additives would provide enhanced chemical, ballistic, rheological and conductive properties as well as greater stability for storage or use at elevated temperatures. Further, the additives would sequester transition metal contaminants that may destabilize the liquid composition, resulting in undesirable off-gassing or premature decomposition, and increase hazard characteristics such as sensitivity to impact or friction. Moreover, the additives provide a pathway to introduce non-polar compounds to the generally polar liquid composition, which impart desired burning rates, ignitability improvement, flame spreading, gas output, and other benefits, which otherwise would not be available due to immiscible behavior. Electrical ignition, combustion adjustment via power controls, modulation of gas generating quantities via flow control techniques of the liquid, all these capabilities exist to advance the science of propulsive performance singly and in combination, which do so without combustion catalysts or pyrotechnic igniters separately employed to assist in the ignition or steady-state combustion of liquid propellants. Finally, the liquid composition would allow the addition of nano-engineered fuel additives (particulate modifiers) to achieve very high burning rates and other aspects of energy management for use in gas generators or propellants. The present embodiment overcomes prior art shortcomings by accomplishing these critical objectives.